It is well known that NOx (nitrogen oxides) emissions in internal combustion engine exhaust can be detrimental to the environment and, thus, are regulated by the government. In order to decrease the NOx emission in exhaust, engine exhaust purification systems are generally equipped with a NOx catalyst to reduce NOx in the exhaust gases to N2 (nitrogen) prior to being emitted into the atmosphere. In a lean environment, meaning an environment where the fuel to air ratio is low, such as in diesel engine exhaust, zeolites containing a range of transition, alkali, alkaline earth, and lanthanide metals have been utilized as catalysts for lean NOx reduction with propene, alcohol, methane or diesel fuel being the reductants. The zeolite is loaded with a metal cation of the groups listed above so the metal cation can compensate for a net negative charge of the zeolite. For instance, exhaust gas purifying catalyst systems, such as that shown in U.S. Pat. No. 5,727,385 issued to Hepburn on Mar. 17, 1998, include a transition metal ion-exchanged zeolite acting as the lean-burn NOx catalyst.
Over the years, chemists have found that zeolites with higher metal loadings, particularly loadings that exceed stoichiometry, possess increased activity and durability for lean NOx reduction. The zeolite exceeds stoichiometry when the exact charge balance between the net negative framework charge and the charge compensating cations is exceeded by the loading of the cations, often resulting in excess cations on outer surfaces of the zeolite. The higher metal loadings act to reduce metal cation migration from active sites of the zeolite. Further, the higher metal loadings provide higher density of the zeolite active sites, resulting in increased activity for lean NOx reduction.
Although higher loadings of the metal cation on the zeolite increases the activity and durability of the zeolite, the higher metal loadings lead to increased formation of sulfate (SO4−2). The excess metal cations are a large contributor to the oxidation of sulfur dioxide (SO2), present in exhaust gas, to form sulfur trioxide (SO3) and sulfuric acid (H2SO4). Sulfate formation deteriorates the purification capability of the NOx catalyst by sitting on the catalyst's active sites. Moreover, sulfate contributes to formation of particulate matter by serving as a nucleic site for the formation of particulate matter. Further, sulfate, upon which particulate matter does not form, may form sulfuric acid and enter the environment, eventually contributing to acid rain.
The present invention is directed to overcoming one or more of the problems set forth above.